The short answer: usually honeydew
If a fine, sticky rain is coating your car, patio furniture or leaves under a tree in the growing season, the most common cause is honeydew. Honeydew is a sugary liquid excreted by sap-feeding insects, mainly aphids and scale insects. They tap into the tree’s phloem, take in more sugar than they need, and the excess drips onto whatever is below.
Honeydew is generally a nuisance rather than a direct threat to a large, healthy tree. It does, however, tell you that insects are feeding on the tree, and it often grows into other problems such as black sooty mold.
How to tell honeydew, sap, resin and gum apart
| Substance | What it is | Where and how it shows up |
|---|---|---|
| Honeydew | Insect excretion | A light, widespread sticky mist or droplets on leaves and surfaces below the canopy, often in late spring through summer |
| Sap | Fluid from the tree’s vascular system | Drips or ooze from a fresh wound, pruning cut or crack, often clear and watery, especially in spring |
| Resin | Thick, aromatic substance from conifers | Sticky blobs on pine, spruce and fir trunks and branches, hardening over time |
| Gum | Thick exudate from stone fruit trees and some others | Amber, jelly-like blobs on cherry, plum and peach trunks, often linked to stress, injury or borers |
If the stickiness is spread on leaves and surfaces beneath the tree, think honeydew. If it is concentrated at a spot on the bark, think sap, resin or gum, which point to a wound or other problem on the tree itself.
Do aphids leave a sticky residue?
Yes. Aphids are small, soft-bodied insects, often green, black, yellow or brown, found in clusters on new shoots and the undersides of leaves. Signs of an aphid problem include:
- Sticky, shiny leaves, and drips onto cars and benches
- Curled or distorted new growth
- Clusters of tiny insects, often with white cast skins
- Ants walking up the trunk, since many ants feed on honeydew and protect aphids
Scale insects and other sap feeders
Scale insects look like small bumps on twigs and leaf veins rather than moving insects, so they are easy to miss. Whiteflies, leafhoppers, psyllids and mealybugs can also produce honeydew.
Sooty mold
Honeydew can grow a black, sooty fungus called sooty mold. It grows on the sugary film, not on the plant tissue itself, and looks like soot on leaves. It can reduce light reaching the leaf, but it usually washes off once the insects are controlled.
Trees where it is common
Honeydew shows up on many species. Maples, lindens (lime trees), crape myrtles, oaks, tulip trees, birches and fruit trees are among those often mentioned. Whether a particular tree is affected can vary from year to year and with local conditions, so seeing it once does not mean it will recur. See are maple trees messy and crape myrtle care for two common cases.
How to treat aphids on trees
In many cases, no treatment is needed. Natural enemies such as lady beetles, lacewings and parasitic wasps often bring aphid numbers down on their own, and a bad year may be followed by a quiet one. If you do want to act, start with the gentlest methods.
- Confirm the cause. Look at the undersides of leaves and new shoots for insects, or check twigs for scale.
- Spray with water. On small trees and shrubs, a strong jet of water can knock aphids off.
- Insecticidal soap or horticultural oil. These products can help against soft-bodied insects when sprayed to contact them. They need to follow label directions, and they can harm some plants or beneficial insects, so read the label.
- Protect natural enemies. Broad-spectrum insecticides can kill the beneficial insects that keep aphids in check.
- Consider a professional for tall trees. Spraying a large tree is hard to do safely and effectively. A certified arborist may recommend systemic treatments applied to the soil or trunk, though these also carry environmental trade-offs, including effects on pollinators when the tree flowers.
Keeping the tree healthy with proper watering, mulch and avoiding over-fertilizing with nitrogen can help, as soft, fast growth tends to attract aphids.
Cleaning sticky residue from cars and patios
- Wash the surface soon after it gets sticky, as dried honeydew is harder to remove
- Use warm soapy water on cars and outdoor furniture; for stubborn spots, use a product made for car or surface care and follow its instructions
- Park away from the tree during the season if possible
- Hosing down paving and decks regularly can help prevent buildup
What about spiles and collecting sap?
A spile is a tap inserted into a tree, mostly used by maple syrup makers to collect sugary sap in late winter and early spring. They do work to collect sap flow, which is the point of the tool, but the sap is not a general source of drinking water, and tapping should be done correctly to limit harm to the tree. If you are interested in tapping, follow guidance on tree size, tap number and timing from a maple producers’ association or extension service. Sap that flows from a wound or pruning cut in spring is a normal response and usually stops on its own.
When sticky ooze signals a bigger problem
Honeydew on its own is rarely an emergency. Take a closer look if you see:
- Sticky oozing or dark bleeding from the trunk, which can be linked to borers or disease, as discussed in holes in a tree trunk
- Branch dieback, thinning leaves or unusual color changes
- Heavy infestations year after year
- Orange or discolored growths on the bark (see orange dots and fungus on trees)
A certified arborist or your local extension service can identify the insect or problem and suggest a plan suited to your region.